Flow divider

ABSTRACT

Flow dividers for dividing a hot gas flow into serveral partial flows through parallel internal channels are used in heat exchangers and as catalyst carriers, and can be produced by shaping the flow divider from metallic aluminium through extrusion or winding, then converting it to hydrated aluminium oxide through anodic oxidation while it is slowly moving down into an electrolyte (4), and finally converting it to alpha-alumina through heat treatment.

BACKGROUND OF THE INVENTION

In power engineering it is a common goal to achieve within a limitedvolume a transfer of energy between a gas flow and a solid body or achemical reaction in a gas aided by a catalyst fixed to the surface of asolid body. The solid body must then be shaped with a maximum contactsurface with the gas flow without too high flow resistance. The solidbody is then often shaped with a large number of parallel channels,separated by thin walls, thereby dividing the gas flow into a largenumber of partial flows with turbulent flow pattern.

Examples of use of such flow dividers are found among heat exchangers,mufflers, catalyst carriers for chemical industry and for emissioncontrol in vehicles. Other examples are for gas flow direction infurnaces, burners and wind tunnels.

For use at high temperatures above 700 degrees C. two types of materialare mainly used, none fully satisfactory.

Ceramic materials, such as aluminium oxide, can be extruded as a slurryto form bodies with parallel channels as disclosed in patents EP 294.106and EP 275.162, and thereafter be converted by heat treatment towater-free alumina, but have disadvantages due to built-in stresses,fragility, difficulty in handling prior to the heat treatment anddifficulty to shape the channel entries for low flow resistance. Theymay need complex mounting devices for enclosure in metal, as shown inU.S. Pat. No. 3,966,419.

Metals are easy to shape, both by extrusion and by winding togethergrooved, corrugated or pleated strips as shown in U.S. Pat. No.4,719,680, but at elevated temperatures they are mechanically unstableby creep deformation, and chemically unstable by reaction with thegases.

SUMMARY OF THE INVENTION

The present invention concerns a method of making flow dividers, wherethe shaping is done in a metallic state, and a conversion to ceramicmaterial is carried out after the shaping. This results in greaterliberty in choice of shape, and lower cost.

It is well known that anodic oxidation in an electrolyte containingoxalic acid permits conversion of the whole thickness of a thin-walledaluminium object to hydrated aluminium oxide. It is also known that heattreatment of hydrated aluminium oxide can convert it to non-hydratedaluminium oxide in the alpha-alumina modification, which is tough andwear resistent. It is also known to combine these known steps tofabricate simply shaped items such as loudspeaker membranes fromalpha-alumina, according to the patent applications DE 35 42 202 and DE35 46 548.

The shapes of metallic flow dividers, extruded as well as wound,comprise portions with larger thickness than average, such as where twowalls meet, and narrow passages where the current density and theoxidation rate are less than average, such as where two layers touchwhen wound together, or far into the central channels. If such bodiesare anodized with the known methods, it would be impossible to avoidmetallic remnants where the thickness is large or the oxidation ratelow, because these portions lose contact with the current source whenthe thinner easily oxidated portions have been converted. During asubsequent heat treatment, the metal would melt and damage the shape ofthe body.

According to the invention, the flow divider with parallel channels isshaped in a first step by extrusion or by winding together corrugated,pleated or grooved strips. If desired, the mechanical stability of awound divider can be improved by thermal bonding in heated inert gas.

In a second step, the channel exits or entrances are chemically deburredto lower the flow resistance.

In a third step, the aluminium metal is converted to hydrated aluminiumoxide through anodic oxidation in an electrolyte containing oxalic acid,with a current source connected to the upper end of the flow divider andthe flow divider is slowly lowered into the electrolyte, with thechannels vertically oriented while the current flows. All parts of theflow divider are then successively converted to hydrated aluminiumoxide, with the not yet converted metal below the surface of theelectrolyte occurring as a middle layer with downwards taperinglydecreasing thickness as shown in FIG. 1. At thicker or slowly oxidizedportions, the taper will reach farther down below the surface of theelectrolyte, at easily oxidized portions not so far. There is no risk offormation of non-converted metallic remnants. When the desired lengthfor the divider has been converted, the current source is disconnected.

In a fourth step, the flow divider is converted from hydrated aluminiumoxide to water-free alpha-alumina through heat treatment at a hightemperture exceeding 700 degrees C. If the flow divider is to be used asa catalyst carrier, the catalyst is thereafter applied in one or moresteps.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Drawings:

FIG. 1 is a diagrammatic vertical cross-sectional view of apparatus forcarrying out the anodic oxidation of a metallic aluminium extruded partto hydrated aluminum oxide in accordance with the method of the presentinvention, and

FIG. 2 is a simplified perspective view of a preferred product producedby the method of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows in vertical cross-section how the slow immersion during athird step of the method that is disclosed in more detail below ensuresthat lower parts are first converted from metallic aluminum to hydratedaluminum oxide and no metallic remnants are left, even at thicker orslowly oxidized portions. The current source 3 has one of its terminalsconnected to the electrolyte 4 and its other terminal to the upper endof the metallic preformed aluminium flow divider 5 which is shown incross section in FIG. 1. Below the surface 2 of the electrolyte, thethickness of the not yet converted aluminium layer in the middle of thewalls is taperingly decreasing, such that it has its original thicknessat the surface 2 of the electrolyte and below this decreases tovanishingly thin at that level 1 where it has been subject to oxidationlong enough for the whole thickness to be converted.

FIG. 2 shows a flow divider which can be manufactured through extrusion.The invention is not limited to this shape of the channels, and alsoincludes flow dividers shaped by winding. The invention is not limitedto items with cylindrical outer surfaces.

In practicing a preferred embodiment of the method of the presentinvention, an elongated body of aluminium metal is formed throughextrusion, with a large number of parallel internal channels, separatedby thin walls. Suitable dimensions are for the wall thickness 0.1 to 0.5mm, and for the channel width 1 to 4 mm, where the smaller dimensions inboth instances refer to catalyst carriers, and the larger dimensions inboth instances refer to heat exchangers. As an alternative, the body canbe formed by winding together one or more aluminium strips, at least oneof them with a corrugated, pleated or grooved surface.

This body is cut to a length somewhat exceeding the desired length ofthe flow divider, is then thoroughly cleaned, whereafter one end isdipped in an alkaline etching solution to deburr and round the channelentrances. Special shaping to facilitate later enclosure or attachmentof the flow divider can be made at this stage.

The other end of the body is connected to a current source, the otherterminal of which is connected to an electrolytic bath, containingoxalic acid, and the body is then slowly lowered into the electrolytewhile the current flows. The aluminium metal is then, through anodicoxidation, converted to hydrated aluminium oxide, starting at the lowerend, the wall thickness becoming larger than before the conversion, andwound layers in contact becoming permanently joined.

When a long-enough piece of the body has been converted, it is removedfrom the electrolyte. Remaining metallic portions at the upper end arecut away. The body is then heated in an oven (not shown) to atemperature above 700 degrees C. The choice of temperature and heatingtime depends on the shape of the body and the type of oven. Ovens withmicrowave heating will require temperatures up to 1100 degrees and timesup to 30 minutes; convection ovens or radiation ovens will need highertemperatures or longer time. This heating stop converts the hydratedaluminium oxide to water-free alpha-alumina.

If the body is to be used as a catalyst carrier, the channels are coatedwith catalyst material after it has cooled.

Flow dividers according to the invention can be used as catalystcarriers in exhaust systems or chemical industry, as heat exchangers andas flow equalizers in convection ovens. The invention is not restrictedto cylindrical shape or to the use within certain temperature ranges. Byperforming the shaping on a metallic body, it can be fitted to differentshape requirements. The good chemical resistance of alpha-alumina makesthe invention useful also in applications where an aggressive chemicalenvironment is encountered.

We claim:
 1. A method for producing a multi-channel gas flow dividermade of alpha-alumina, comprising:(a) forming out of metallic aluminum abody of generally constant transverse cross-sectional shape throughoutthe longitudinal extent thereof between two opposite ends thereof, sothat the body has a plurality of internal channels extendinglongitudinally thereof between said ends and has, within said transversecross-sectional shape, some regions which have a greater effective wallthickness than others; (b) while supporting said body in an uprightposition so that one said end is lowermost, progressively lowering saidbody into a bath of electrolyte while applying opposite electricalcharges to:(i) said body from above said bath, and (ii) said electrolytein said bath in such a sense as to thereby cause longitudinally andthicknesswise progressive anodization of said metallic aluminum tohydrated aluminum oxide from externally of said lowermost end, upwardsand inwards to such an extent that said metallic aluminum issubstantially fully converted to hydrated aluminum oxide throughout aportion of the longitudinal extent thereof extending upwards from saidlowermost end; and (c) thereafter converting said hydrated aluminumoxide in said portion to alpha-alumina by heating said portion to atemperature exceeding 700° C.
 2. The method of claim 1, wherein:saidstep of forming comprises extruding a shape from metallic aluminum,cutting said shape to a desired length to provide said body, anddeburring said channels at said one end of said body prior to conductingstep (b).
 3. The method of claim 1, wherein:said step of formingcomprises winding into a multiple-layer coil a corrugated strip ofmetallic aluminum and thereby creating said channels.
 4. The method ofclaim 3, wherein:said step of forming further comprises thermallybonding said layers together.
 5. The method of claim 4, wherein:saidstep of forming further comprises deburring said channels at said oneend of said body prior to conducting step (b).
 6. The method of claim 1,wherein:said step of forming comprises winding into a multiple-layercoil a pleated strip of metallic aluminum and thereby creating saidchannels.
 7. The method of claim 6, wherein:said step of forming furthercomprises thermally bonding said layers together.
 8. The method of claim7, wherein:said step of forming further comprises deburring saidchannels at said one end of said body prior to conducting step (b). 9.The method of claim 7, wherein:said step of forming comprises windinginto a multiple-layer coil a grooved strip of metallic aluminum andthereby creating said channels.
 10. The method of claim 9, wherein:saidstep of forming further comprises thermally bonding said layerstogether.
 11. The method of claim 10, wherein:said step of formingfurther comprises deburring said channels at said one end of said bodyprior to conducting step (b).
 12. The method of claim 1, furthercomprising:between steps (b) and (c), severing said portion from saidbody.
 13. A multi-channel gas flow divider produced by the process ofclaim 12.